1. Field of the Invention
The present invention relates to a semiconductor laser apparatus, and more specifically to a semiconductor laser apparatus having a plurality of light emitting points.
2. Background Art
Presently, as an optical recording medium, a compact disk (CD), a mini disk (MD), a digital versatile disk (DVD) or the like have been known. In the pickups of these recording media, for example, for replaying a DVD and a CD, laser of an oscillating wavelength of 650 nm for the DVD and laser of a oscillating wavelength of 780 nm for the CD are required for a light source. Furthermore, in order to realize the simplification or miniaturization of an optical pickup, a monolithic laser apparatus that can output both wavelengths of 650 nm and 780 nm from a chip is effective.
In the case of laser having a plurality of light emitting points, a structure wherein a plurality of light emitting points are disposed at even intervals or in left-right symmetry to the center of the chip is generally used. For example, in the case of a semiconductor laser having two light emitting points for CD/DVD and a chip width of 300 μm, in general, the two light emitting points are disposed in left-right symmetry, and the light emitting point of 650 nm—LD is generally controlled so as to locate at 55 μm from the center (18.3%). When the chip width is changed to 250 μm and 200 μm, the light emitting point of 650 nm—LD is also generally controlled so as to locate at 55 μm from the center (22% and 27.5%).
In a laser having the locations of light emitting points of such percentages to the width of the chip, since stress to light emitting points is different depending on left and right of each light emitting point, the elements fabricated from the same material using the same procedures have different characteristics depending on the locations of the light emitting points. When the locations of two light emitting points are asymmetry to the center of a chip, stress to the light emitting points differs depending on the distance from the center of the light emitting points to the width of the entire chip, and affects the light emitting characteristics.
Stress applied to a light emitting point becomes larger as the light emitting point departs from the center of a chip. Therefore, at the light emitting point apart from the center of a chip, the asymmetry of stress applied to the left and the right of the light emitting point cannot be negligible. For example, at the light emitting point greatly apart from the center of a chip to the right, strong stress is applied to the right of the light emitting point, and at the light emitting point greatly apart from the center of a chip to the left, strong stress is applied to the left of the light emitting point; therefore, the characteristics of the left light emitting point becomes different from those of the right light emitting point. In particular, when an off substrate (tilted substrate) is used, or when a plane close to a light emitting point is used as a die-bonding plane, since a solder material is used to bond the chip to a mounting point, such as a stem, the applied stress becomes larger, and asymmetry of the stress applied to the light emitting point becomes significant.
Particularly in the case of a simple ridge stripe structure wherein the both ends of the ridge stripe structure are not buried with a semiconductor material, since such stress is strong, the effect of the asymmetry is great.
As described above, stress applied to a light emitting point changes depending on the distance from the center of the chip to the light emitting point. For example, it has been clarified from experimental facts that when the surface close to the light emitting point of a double wavelength laser having a chip width of 200 μm, and a light emitting point of 650 nm—LD at the location of 55 μM from the center of the chip is die-bonded on the die bonding plane using a solder or the like, in optical properties, the polarization angle is less than −10°, and the polarization ratio decreases to about 60.
There is limitation in the distance between two light emitting points depending on the wavelength band to be used. For example, in a generally used 780 nm/650 nm double wavelength semiconductor laser apparatus for CD/DVD, a distance of about 110±10 μm is required between the two light emitting points. Therefore, all the light emitting points cannot be disposed in the vicinity of the center of the chip to minimize the effect of stress applied to the light emitting points.